1. Field of the Invention
This invention relates to a valve timing control system for an internal combustion engine, which varies the cam phase of at least one of an intake cam and an exhaust cam, relative to a crankshaft of the engine, to thereby control valve timing.
2. Description of the Prior Art
Conventionally, a valve timing control system of the above-mentioned kind was proposed e.g. in Japanese Laid-Open Patent Publication (Kokai) No. 9-217609. In this control system, a cam phase change mechanism supplied with hydraulic pressure controlled by an hydraulic pressure control valve changes the cam phase by changing the angle of a camshaft relative to a cam pulley. The hydraulic pressure control valve formed by a linear solenoid valve includes a coil and a spool driven by a force generated by the coil. The output duty factor of current supplied to the coil is controlled to drive the spool to a position corresponding to the output duty factor, i.e. the amount of current supplied to the coil, whereby hydraulic pressure is selectively supplied to an advance chamber or a retard chamber of the cam phase change mechanism, to drive the cam phase in an advancing or retarding direction. Further, when the output duty factor is controlled to a hold duty factor value approximately in the center of a control range thereof, the spool is controlled to a neutral position for simultaneously closing the advance chamber and the retard chamber, thereby cutting off supply of the hydraulic pressure to both of the chambers. This holds the cam phase. Further, in this control system, the output duty factor is feedback-controlled such that an actual cam phase detected becomes equal to a desired cam phase set in dependence on operating conditions of the engine.
The control system, however, suffers from a problem that the cam phase cannot be controlled with accuracy when the temperature condition of the hydraulic pressure control valve is changed. More specifically, in the linear solenoid valve which is used in the control system as a hydraulic pressure control valve, the resistance of the coil varies with its temperature, so that the amount of current actually flowing through the coil varies even if the output duty factor remains the same. For instance, under a low temperature condition of the coil, the resistance of the coil is small, so that even if the output duty factor remains the same, the amount of current actually flowing through the coil increases. This increase in the current amount reduces the hold duty factor value, thereby causing the whole control range of the output duty factor to shift in the direction of a lower output duty factor, and at the same time increases a change in hydraulic pressure per unit change in the output duty factor (i.e. increases sensitivity of the hydraulic pressure control valve), resulting in an inevitable decrease in the controllable range of the output duty factor. On the other hand, under a high temperature condition of the coil, the resistance of the coil increases, so that the amount of current flowing through the coil increases even if the output duty factor remains the same. This increases the hold duty factor value, thereby causing the whole control range of the output duty factor to shift in the direction of a higher output duty factor, and at the same time reduces a change in hydraulic pressure per unit change in the output duty factor (i.e. decreases sensitivity of the hydraulic pressure control valve), resulting in an increased controllable range of the output duty factor and enhanced control accuracy.
In spite of this problem, the above conventional control system simply controls the amount of current supplied to the coil of the hydraulic pressure control valve by the output duty factor calculated based on the desired cam phase and the actual cam phase by feedback control without further processing. Therefore, even if the output duty factor is calculated such that the optimum cam phase corresponding to the present operating condition of the engine can be obtained, the behavior of the hydraulic pressure control valve and that of the cam phase change mechanism controlled thereby are varied depending on the actual coil temperature due to the above temperature characteristics of the control valve, which prevents control of the cam shaft to an intended cam phase, thereby making it impossible to perform accurate cam phase control.
To solve such a problem, it is contemplated, for instance, that the actual temperature of the coil is detected to correct the output duty factor based on a result of the detection. In this case, however, a temperature sensor for detecting the coil temperature is additionally required. Further, in general, temperatures are slow in change, and the temperature of the coil largely depends on environments surrounding the coil, such as the temperature within an engine room of an automotive vehicle on which the control system is installed, wind generated by running of the vehicle, and heat generated in the coil by current flowing therethrough. This makes it difficult to accurately estimate the amount of current which is actually flowing through the coil at the time of detection of the coil temperature, based on the detected coil temperature or compensate for variation therein. As a result, the cam phase cannot be controlled with accuracy.
It is an object of the invention to provide a valve timing control system for an internal combustion engine, which is capable of properly controlling a control valve for control of a cam phase irrespective of temperature conditions of a coil of the control valve, thereby enhancing the accuracy of feedback control of the cam phase.
To attain the above object, the present invention provides a valve timing control system for an internal combustion engine, which includes a crankshaft, an intake valve, an exhaust valve, an intake cam, and an exhaust cam, and controls valve timing of at least one of the intake valve and the exhaust valve, by changing a cam phase which is a phase of at least one of the intake cam and the exhaust cam, relative to the crankshaft.
The valve timing control system according to the invention is characterized by comprising:
a cam phase change mechanism for changing the cam phase;
a control valve having a coil, for driving the cam phase change mechanism according to an amount of current flowing through the coil;
actual cam phase-detecting means for detecting an actual cam phase;
desired cam phase-setting means for setting a desired cam phase depending on operating conditions of the engine;
cam phase feedback control means for feedback-controlling a control value for control of the amount of current such that the actual cam phase becomes equal to the desired cam phase;
desired current amount-setting means for setting a desired amount of current based on the control value controlled by the cam phase feedback control means;
actual current amount-detecting means for detecting an actual amount of current actually flowing through the coil of the control valve; and
current feedback control means for feedback-controlling an output control value for control of the amount of current supplied to the control valve such that the actual amount of current becomes equal to the desired amount of current.
According to this valve timing control system, a control value used for controlling the amount of current flowing through the coil is feedback-controlled such that the actual cam phase becomes equal to the desired cam phase. Further, a desired amount of current is set based on the control value controlled by the feedback control, while an actual amount of current flowing through the coil of the control valve is detected. An output control value for control of the amount of current supplied to the control valve is feedback-controlled such that the actual amount of current becomes equal to the desired amount of current. This causes current to be supplied to the control value in an amount corresponding to the calculated output control value, whereby the amount of current flowing through the coil is properly controlled.
As described above, according to the invention, the valve timing control system carries out not only cam phase feedback control in which the control value for control of the amount of current supplied to the control valve is feedback-controlled such that the actual cam phase becomes equal to the desired cam phase, but also current feedback control in which the output control value for finally controlling the amount of current supplied to the control valve is feedback-controlled such that the actual amount of current flowing through the coil of the control valve becomes equal to an optimum desired amount of current set based on the control value calculated by the cam phase feedback control. Thus, the actual amount of current flowing through the coil is directly detected, and the output control value is feedback-controlled such that the actual amount of current becomes equal to the optimum desired amount of current. This makes it possible to cope with all the temperature conditions of the coil, so as to suitably compensate for variations in the behavior of the control valve, caused by changes in temperature of the coil. Therefore, it is possible to carry out optimum control of the operation of the control valve and that of the cam phase change mechanism irrespective of the temperature conditions of the coil, thereby enhancing accuracy of the cam phase feedback control.
Preferably, the control value and the output control value are values of an identical kind of control amount, and a range of values of the identical kind of control amount within which the output control value can be set is wider than a range of values of the identical kind of control amount within which the control value can be set.
More preferably, the identical kind of control amount is a duty factor of output of the current supplied to the coil.
Preferably, the desired current amount-setting means includes a conversion table for converting the control amount to the desired amount of current.
More preferably, the conversion table represents an optimum relationship between the control value and the desired amount of current obtained by the control value, under a normal temperature condition of the coil.
The above and other objects, features, and advantages of the invention will become more apparent from the following detailed description taken in conjunction with the accompanying drawings.